Cut each sunchoke in half lengthwise, and then the bigger pieces into fourths. Place the sunchokes, turnip, and beets in a single layer in a heavy, metal roasting pan. Sprinkle with salt, pepper, and olive oil. Toss to coat.

In a small bowl, mix the garlic, tarragon, dill, and mustard. Pat evenly over the top and sides of the roast. Set aside.

Place the roast over the vegetables. Bake, uncovered, 1 hour 45 minutes.

Remove the roast from the pan. Add the grapes, shallots, vinegar, and chicken stock to the pan, stirring or swirling to pick up the fond that the meat left behind. Return the uncovered pan with sauce and vegetables to the oven for 5 minutes. Slice the meat, place on platter, surround with the vegetables. Spoon the pan sauce over the meat.